Router jigs that make dovetails for woodwork generally consist of a template having a row of finger-like projections that form the top surface of the jig. A router, used as a cutting means, sits upon the template such that the spinning cutter thereof is guided in and out of the finger-like projections by a guide bush encircling the cutter. The workpiece is fixed below the template to be cut. This system has several advantages. It is easy to understand as a concept, and requires little skill to operate. Unfortunately, it commonly produces very mechanical looking dovetails with the dovetail appearing the same size as the dovetail pin. The template is quite easily damaged by the router through inattention of the operator, and in practice, it is difficult to cut the wood without the cutter causing breakout or tear out of the wood fibers.
Two basic types of dovetail joint can be made with jigs. The first type is referred to as a through dovetail, which is often used for jointing boards to form chests and boxes. Both the tail and the dovetail pin show end-grain. For through dovetails, the two pieces are cut one after the other, with both components being held in the device vertically. The first piece is cut with a dovetail cutter to make the dovetail sockets. The second piece is shaped with a straight cutter moving at angles to make wedge shaped pins to fit into the sockets. The angle of the pin is determined by the angle of the template's fingers which is made to match the angle of the cutter that cuts the socket. The second type is referred to as a half-blind or lapped dovetails, which is commonly used for drawer fronts where the joint is visible from the side, but no end-grain shows on the front of the drawer. For lapped dovetails, the two pieces to be joined are mounted with one piece vertical and the other horizontal. The cutter cuts through the vertical piece making tail sockets and goes a set distance into the other horizontal piece making the pin pockets. The spacing of the cuts and the forward travel of the cut into the second horizontal piece are controlled by the length and spacing of the fingers of the template. The depth of the cut vertically is determined by the depth of the router. Both the tail and the pin are made by the same dovetail cutter at the same time. Simple jigs are so designed that the socket made by the cutter and the wood left between sockets are equal. By this means, the wooden dovetail shape between two sockets of one piece is fitted into the socket of the other and vice versa. Each template is matched to a particular cutter size. Fine adjustment of the fit is obtained by fractionally raising or lowering the cutter.
Mostly, these two different joints are made on different jigs. There are types of jigs that combine the two, but the principle of fixing the wood and moving the router in and out of the fingers of a template is common to all types, with one significant exception.